This one was reported to me by a user on Reddit. Trying to sideload games via https://play.date/account/sideload/ fails with a 500 if the game has a "%" character in the description field. I assume there are other triggers, but removing that character fixed it in my case. Here's a zip that will trigger the error:
Heh. If they're using Python and the ConfigParser module, then replacing the '%' with '%%' might also work.
And yep, I can reproduce the bug on my linux box, getting a nice big phatt 'SERVER ERROR' message on the upload screen. But if I replace the '%' with '%%' it works just fine. I dunno if it will display as a '%', but I'd bet money it will.